Flywheels are well known devices, used for storing energy in a rotating mass. The amount of energy stored in a flywheel is proportional to the square of its rotational speed. In general, energy is transferred to a flywheel for storage by the application of a torque to the flywheel, causing its rotational speed to increase. Conversely, energy can be released or recovered from a flywheel by the flywheel applying a torque to a load, as a result of which the flywheel's rotational speed decreases.
Many known flywheel kinetic energy storage assemblies include an electrical machine which can function either as a motor or a generator. When the electrical machine acts as a motor (i.e. when the flywheel assembly is in “motoring” or “recuperating” mode), electrical energy supplied to the machine is converted to kinetic energy and, as a result, the flywheel mass rotates more quickly. When the electrical machine acts as a generator (i.e. when the flywheel assembly is in “generating” or “boosting” mode), kinetic energy stored in the flywheel mass is converted to electrical energy and can be supplied onwards to another component with a system such as an electric motor and as a result the flywheel rotates more slowly.
In practice, the maximum speed at which a flywheel mass can safely rotate, and hence the amount of energy which the corresponding flywheel assembly can store, will depend at least in part on the mechanical strength and strain capabilities of the flywheel assembly. For example, an important factor is its behaviour in response to the mechanical stresses which are experienced at high rotational speeds.
When a flywheel assembly comprises an electrical machine, another factor in how fast the flywheel mass can rotate is the manner in which the electrical machine can be controlled. It is desirable to implement a reliable and efficient control scheme, which works at a range of flywheel rotational speeds. It is also desirable to avoid and/or reduce losses, for example losses due to heat dissipation, as much as possible. And it is important that the flywheel assembly is as safe as possible, including being safe in the event that the flywheel assembly experiences a failure or breakage.
According to known methods, the manner in which the rotating parts of a flywheel assembly are to be controlled can have a significant influence on the manner in which the physical components of the flywheel assembly are constructed. For example, the magnetic rotating part(s) of the electrical machine can be shaped in a particular manner in order to determine their magnetic properties, in preparation for application of electrical pulses to the flywheel assembly in a particular pattern such as a sine wave.
GB 1312924.2 filed 19 Jul. 2013 and GB1312927.5 filed 19 Jul. 2013 are hereby incorporated by reference in their entirety.
An invention is set out in the claims.
According to an aspect there is provided an annular rotor for a flywheel as defined in claim 1. Thus there is provided an annular rotor for a flywheel, the rotor comprising a tow comprising a matrix of fibres wound about an axis and arranged in layers comprising a gap between adjacent tow windings, wherein the winding angle between a normal to the axis and the tow is less than 3.5° and the matrix further comprises magnetic particles of a size less than the critical flaw size of the tow.
Optionally, the winding angle is less than 1.5°.
Optionally, the winding angle is less than 0.6°.
Optionally, the winding angle is greater than 0.3°.
Optionally, the magnetic particles are non-spherical in shape.
Optionally, the magnetic particles are substantially linear in shape.
Optionally, the magnetic particles are less than 500 μm in length.
Optionally, the magnetic particles are more than 10 μm in length.
Optionally, the density of the magnetic particles reduces with increasing distance from the axis.
Optionally, the magnetic particles comprise quench milled NdFeB.
Optionally, the tow comprises unidirectional fibres.
Optionally, the annular rotor wherein magnetic particles of longest dimension less than 70 μm are positioned in the gap between coincident tows.
Optionally, the rotor is magnetised.
Optionally, the annular rotor comprises north-south pole pairs.
Optionally, the north-south pole pairs are alternating north-south pole pairs, each pole of a pair occupying an arc around the annular rotor.
Optionally, the annular rotor comprises 12 north-south pole pairs.
Optionally, the pole pairs are arranged to direct a majority of magnetic flux towards the axis of the annular rotor.
Optionally, the annular rotor is an inner portion; and further comprising an outer portion comprising a matrix of fibres wound about the same axis, the outer portion having a diameter larger than the inner portion and wherein the winding angle between a normal to the axis and the tow of the outer portion is 1° or less.
Optionally, an area of the outer portion in alignment with each north pole of the inner portion is painted black or white and an area of the outer portion in alignment with each south pole of the inner portion is painted the other of black or white respectively.
Optionally, the rotor is housed within a vacuum.
Optionally, the rotor is the rotor of a flywheel.
According to a second aspect there is provided a method of providing an annular rotor for a flywheel as defined in claim 11. Thus there is provided a method of providing an annular rotor for a flywheel comprising winding a tow comprising a matrix of fibres about an axis, the tow being arranged in layers comprising a gap between adjacent tow windings, wherein the winding angle between a normal to the axis and the tow is less than 3.5° and providing magnetic particles within the matrix of a size less than the critical flaw size of the tow.
Optionally, the method wherein the winding angle is less than 1.5°.
Optionally, the method wherein the winding angle is less than 0.6°.
Optionally, the method wherein the winding angle is greater than 0.3°.
Optionally, the method wherein the magnetic particles are non-spherical in shape.
Optionally, the method wherein the magnetic particles are substantially linear in shape.
Optionally, the method wherein the magnetic particles are less than 500 μm in length.
Optionally, the method wherein the magnetic particles are more than 10 μm in length.
Optionally, the method wherein the density of the magnetic particles reduces with increasing distance from the axis.
Optionally, the method wherein the magnetic particles comprise quench milled NdFeB.
Optionally, the method wherein the tow comprises unidirectional fibres.
Optionally, the method wherein the magnetic particles are urged to lay flat in the direction of the winding on the surface of the individual tow layers.
Optionally, the method wherein particles of size less than 70 μm are urged to collect in the gap between coincident tows when winding the annular rotor.
Optionally, the method wherein after winding the tow, the step of imparting flux onto the annular rotor is performed so as to magnetise the annular rotor.
Optionally, the method wherein rotor comprises north-south pole pairs
Optionally, the method wherein the magnetised rotor comprises alternating north-south pole pairs, each pole of a pair occupying an arc around the annular rotor.
Optionally, the method wherein the rotor comprises 12 north-south pole pairs.
Optionally, the method wherein the pole pairs are arranged to direct a majority of magnetic flux towards the axis of the annular rotor.
Optionally, the method wherein annular rotor is an inner portion and further comprising winding an outer portion comprising a matrix of fibres wound about the same axis, the outer portion having a diameter larger than the inner portion and wherein the winding angle between a normal to the axis and the tow of the outer portion is 1° or less.
Optionally, the method further comprising painting an area of the outer portion in alignment with each north pole of the inner portion black or white and painting an area of the outer portion in alignment with each south pole of the inner portion the other of black or white respectively.
In the figures, like elements are indicated by like reference numerals throughout.